1. Field of the Invention
The present invention relates to non-volatile memory, and more particularly, to a non-volatile memory including sub cell arrays respectively including X-decoders/drivers for simultaneously writing data values, and a method of writing data to the non-volatile memory.
2. Description of the Related Art
Next generation memory demands highly integrated dynamic random access memory (DRAM), non-volatile flash memory and high-speed static random access memory (SRAM). Currently, phase-change random access memory (PRAM), nano-floating gate memory (NFGM), polymer random access memory (PoRAM), magnetic random access memory (MRAM), ferro-electric random access memory (FRAM), resistive random access memory (RRAM) and the like are regarded as next generation memory which meet the above demands.
FIG. 1 is a schematic view of a cell structure of a conventional bi-directional RRAM.
Referring to FIG. 1, the bi-directional RRAM includes a non-ohmic device and a resistance variable device. In the RRAM, data is written using resistance-value variations of the resistance variable device. The resistance variable device includes a resistance variable substance between first and second electrodes.
The resistance value of the resistance variable substance varies in accordance with an applied voltage or an applied current. In uni-directional RRAM, the resistance value varies in accordance with the amount of applied voltage or applied current. In comparison, in the bi-directional RRAM, the resistance value varies in accordance with the amount and the direction of applied voltage or applied current.
The bi-directional RRAM illustrated in FIG. 1 realizes bi-directivity as described above by including the non-ohmic device. The non-ohmic device is in a high-resistant state in a predetermined voltage range VNO− to VNO+ (e.g., −3V to 3V). Accordingly, a current is not applied to the resistance variable device. On the other hand, the non-ohmic device is in a low-resistant state outside of the predetermined voltage range of −3V to 3V. Accordingly, a current is applied to the resistance variable device. U.S. Pat. No. 6,909,632 discloses an example of a bi-directional RRAM including a non-ohmic device and a resistance variable device in more detail.
FIG. 2 is a graph illustrating cell characteristics of the conventional bi-directional RRAM illustrated in FIG. 1.
Referring FIGS. 1 and 2, when a writing voltage VW of 6V is applied to the resistance variable substance, a corresponding cell has a first resistance. In comparison, when a writing voltage −VW of −6V is applied to the resistance variable substance, a corresponding cell has a second resistance.
In the bi-directional RRAM, a data value “1” can be set when a cell has the first resistance and a data value “0” can be set when the cell has the second resistance. That is, in the bi-directional RRAM, the data values “1” and “0” can be written using the writing voltages VW and −VW, wherein the magnitudes of the writing voltages VW and −VW at both ends or terminals of the cell, respectively, are the same but the polarities are different.
FIGS. 3A and 3B are schematic views illustrating operations of writing data to a cell of the conventional bi-directional RRAM illustrated in FIG. 1.
Referring to FIG. 3A, a data value “0” is written to a cell (indicated by a circle) by applying 3V to a word line WL and −3V to a bit line BL. On the other hand, a data value “1” is written to the cell by applying −3V to the word line WL and 3V to the bit line BL. Here, 0V is respectively applied to an unselected word line WL′ and an unselected bit line BL′.
Referring to FIG. 3B, a data value “0” is written to a cell by applying 6V to a word line WL, 0V to a bit line BL, and 3V respectively to an unselected word line WL′ and an unselected bit line BL′. On the other hand, a data value “1” is written to the cell by applying −6V to the word line WL, 0V to the bit line BL, and −3V respectively to the unselected word line WL′ and the unselected bit line BL′.
However, when data is written by applying the writing voltages VW and −VW (e.g., VW=6V, −VW=−6V) to a word line WL or a bit line BL as shown in FIG. 3B, the voltage of an unselected word line WL′ and an unselected bit line BL′ changes in accordance with the data value. Thus, it is more efficient to write the data by applying ½ writing voltages ½VW and −½VW (e.g., ½VW=3V, −½VW=−3V) to the word line WL or the bit line BL as shown in FIG. 3A.
For convenience of explanation, a bi-directional RRAM that operates as shown in FIG. 3A will be described below.
FIG. 4 is a detailed view illustrating the writing operation illustrated in FIG. 3A.
Referring to FIG. 4, multiple input/output lines IO0 through IO15 commonly include multiple word lines WLi and WLj. That is, all the input/output lines IO0 through IO15 of a memory cell array share one X-decoder and one driver.
However, bias voltages ½VW and −½VW (½VW=3V, −½VW=−3V) having opposite polarities as shown in FIG. 3A cannot be simultaneously applied to one word line, such as the word line WLi. Accordingly, if any of the input/output lines IO0 through IO15, which share the same word line, has a different data value to be written, data cannot be simultaneously written to all the input/output lines IO0 through IO15.